Determining tubing wall thickness

Abstract

A method for monitoring tubing wall thickness includes conveying a tool through a tubular string in a borehole, the tool including a photon source that directs a photon beam along a radial path toward a wall of the borehole, the tool further including an array of collimated detectors that measure Compton backscattering rates at respective distance bins along the radial path. The method further includes calculating a sequence of ratios between measurements from neighboring detectors. The method further includes identifying one or more local extrema in the sequence, each extremum representing a boundary between different materials. The method further includes determining a wall thickness of the tubular string from absolute or relative positions of the one or more extrema. The method further includes displaying a representation of the wall thickness.

What is claimed is: 
     
       1. A method for monitoring tubing wall thickness, the method comprising:
 conveying a tool through a tubular string in a borehole, the tool comprising a photon source that directs a photon beam along a radial path toward a wall of the borehole, the tool further comprising an array of collimated detectors that measure Compton backscattering rates at respective distance bins along the radial path; 
 calculating a sequence of ratios between measurements from neighboring detectors; 
 identifying one or more local extrema in the sequence, each extremum representing a boundary between different materials; 
 determining a wall thickness of the tubular string from absolute or relative positions of the one or more extrema; and 
 displaying a representation of the wall thickness. 
 
     
     
       2. The method of  claim 1 , wherein the representation shows wall thickness as a function of position along the borehole. 
     
     
       3. The method of  claim 2 , wherein the representation shows wall thickness as a function of azimuth around the borehole. 
     
     
       4. The method of  claim 1 , wherein the one or more local extrema comprise a valley representing an inner wall of the tubular string and a peak representing an outer wall of the tubular string. 
     
     
       5. The method of  claim 4 , wherein determining the wall thickness comprises calculating a distance between the valley and the peak. 
     
     
       6. The method of  claim 4 , wherein the tubular string is casing. 
     
     
       7. The method of  claim 4 , wherein the tubular string is surrounded by a casing string, and the one or more local extrema further comprise a valley and peak representing walls of the casing string. 
     
     
       8. The method of  claim 7 , wherein determining the wall thickness comprises calculating a distance between the valley and peak representing walls of the casing string to obtain a wall thickness of the casing string. 
     
     
       9. The method of  claim 1 , wherein the photon source is maintained at a fixed distance from an inner wall of the tubular string, wherein at least one of the one or more local extrema represents an outer wall of the tubular string, and wherein determining the wall thickness comprises converting an absolute position of the at least one extrema to the wall thickness. 
     
     
       10. The method of  claim 1 , wherein the array comprises at least eight collimated detectors. 
     
     
       11. An apparatus for monitoring tubing wall thickness, comprising:
 a photon source to direct a photon beam along a radial path toward a wall of the borehole; 
 an array of collimated detectors to measure Compton backscattering rates at respective distance bins along the radial path; and 
 a processor to calculate a sequence of ratios between measurements from neighboring detectors, identify one or more local extrema in the sequence, each extremum representing a boundary between different materials, and determine a wall thickness of the tubular string from absolute or relative positions of the one or more extrema. 
 
     
     
       12. The apparatus of  claim 11 , wherein the processor outputs for display a representation of the wall thickness. 
     
     
       13. The apparatus of  claim 12 , wherein the representation shows wall thickness as a function of position along the borehole. 
     
     
       14. The apparatus of  claim 13 , wherein the representation shows wall thickness as a function of azimuth around the borehole. 
     
     
       15. The apparatus of  claim 11 , wherein the one or more local extrema comprise a valley representing an inner wall of the tubular string and a peak representing an outer wall of the tubular string. 
     
     
       16. The apparatus of  claim 15 , wherein determining the wall thickness comprises calculating a distance between the valley and the peak. 
     
     
       17. The apparatus of  claim 15 , wherein the tubular string is casing. 
     
     
       18. The apparatus of  claim 15 , wherein the tubular string is surrounded by a casing string, and the one or more local extrema further comprise a valley and peak representing walls of the casing string. 
     
     
       19. The apparatus of  claim 18 , wherein determining the wall thickness comprises calculating a distance between the valley and peak representing walls of the casing string to obtain a wall thickness of the casing string. 
     
     
       20. The apparatus of  claim 11 , wherein the photon source is maintained at a fixed distance from an inner wall of the tubular string, wherein at least one of the one or more local extrema represents an outer wall of the tubular string, and wherein determining the wall thickness comprises converting an absolute position of the at least one extrema to the wall thickness.